This invention relates to an A-D converter in which an input analog signal is converted into a digital signal to obtain digital outputs comprising high-order digits, the digital output is re-converted into an analog signal, the difference between the re-converted analog signal and the input analog signal is amplified and converted into a digital signal to obtain digital outputs comprising low-order digits, so that a highly accurate conversion is performed.
In this kind of A-D converters, for achieving conversion with high accuracy, it is necessary that the D-A converter for re-converting the high-order digit output be stable and highly accurate. However, such a D-A converter requires resistors having high accuracy and stability. At present these are expensive and bulky winding resistors. Additional bulky winding resistors (the same number as the high-order digits to be converted) must also be provided, resulting in the A-D converter becoming costly as a whole. Further, in an amplifier for amplifying the difference between the analog signal re-converted from the high-order digits and the input signal, an offset, that is, an output even in the absence of an input to the amplifier, is caused. If the offset is constant, its influence can easily be removed; but the offset varies with an ambient temperature change and with the lapse of time, that is, the offset drifts, so that an error is introduced in the digital converted output. Moreover, if the gain of the amplifier for amplifying the difference between the input analog signal and the analog converted output of the high-order digit output varies with an ambient temperature change and with the lapse of time, then no correct digital output can be obtained.